Hydrazinium nitroformate

ABSTRACT

The invention relates to crystalline hydrazinium nitroformate having a length to diameter ratio of at most 2.5 and a sensitivity to friction and shock not below 20 N and 2 J, respectively, as well as to a method for the preparation thereof.

The invention relates to crystalline hydrazinium nitroformate, to amethod for the preparation thereof and to the use thereof in solidpropellants, for instance for driving rocket engines and the like.

The use of hydrazinium nitroformate in solid propellant is known fromvarious publications. From U.S. Pat. No. 3,708,359, for instance, asolid propellant composition is known based on hydrazinium nitroformateas an oxidizer and a binder on the basis of a saturated polymerhydrocarbon. The hydrazinium nitroformate is present in the compositionin crystalline form. The crystals are bonded by means of the polymerhydrocarbon.

In the production of the solid substance used in the propellant as acrystalline substance, not only chemical specifications with respect topurity of the substance but also physical properties, such as particleform, average particle size and particle size distribution, are at leastequally important. The fact is that the physical parameterssubstantially determine the bulk density of the product and the shakedensity (or tap density) of the product. The latter measure representsthe density of the product after shaking or tapping a prescribed numberof times. In particular for powdered materials such as hydraziniumnitroformate used as fillers in a polymer matrix for solid propellantsthis measure is important.

A high tap density means that the open space (porosity) between theparticles is small. This open space should be filled with the polymermatrix. This means that the higher the tap density the larger the amountof hydrazinium nitroformate that may be present in the propellant.

For the use of hydrazinium nitroformate in solid propellants it isimportant that a high load of solid substance (filling degree) ispossible. In practice, filling degrees of 80 wt. % or more are normalfor solid propellants. Besides, the rheological behavior of theparticles in the polymer matrix is important. This polymer matrixconsists of a liquid plastic component which is cured after bringinginto the final form. Consequently, efforts are directed, on the onehand, to realizing a highest possible filling degree and, on the otherhand, to still having a mixture capable of being processed into anydesired form (for instance by casting), after which it is cured.

For use in solid propellants hydrazinium nitroformate is oftenrecrystallized after the synthesis to meet the requirements of purityand desired average particle size. In the present production techniques,however, a needle-shaped product is obtained which generally has alength to diameter ratio (L/D value) of at least 4 to 5. It has beenfound that the crystalline substances having such length to diameterratios are hard to process into a propellant with a filling degree of atleast 80 wt. %.

Another aspect of the use of hydrazinium nitroformate is the sensitivityof the product to shock and/or friction. Hydrazinium nitroformate can berapidly decomposed by shock and/or friction. It has been found thatadaptations during the recrystallization process with the purpose ofobtaining a morphology that is more suitable for processing into solidpropellants with a high filling degree may give rise to an increasedsensitivity to shock and/or friction, which involves more risk duringthe processing into solid propellants. It is, for instance, known fromU.S. Pat. No. 3,222,231 to reduce the length to diameter ratio to 1.5 byrecrystallization under ultrasonic treatment. An examination has shownthat such a product has a substantially increased sensitivity to shockor friction as compared with the starting product.

It is an object of the invention to provide a crystalline hydraziniumnitroformate with which, on the one hand, a high filling degree can beobtained, while, on the other hand, the processability of the dispersionof the crystals in the liquid polymer matrix prior to curing is good.

The invention is based on the surprising insight that it is possible toobtain crystalline hydrazinium nitroformate having a length to diameterratio of at most 2.5 and a sensitivity to friction and shock not below20 N and 2 J, respectively.

According to a preferred embodiment of the invention such a crystallinehydrazinium nitroformate is obtainable by pressing crystals having alength to diameter ratio of at least 3 until a particular pressure,thereby obtaining crystalline material that, after the mechanicaltreatment, is still free-flowing or can readily be made free-flowing. Inthis manner, no solid cake of hydrazinium nitroformate is obtained butit appears that a loosely coherent whole is obtained which hassubstantially retained its original diameter but has a considerablyreduced length to diameter ratio.

The mild pressing treatment causes the more or less needle-shapedcrystals to break transversely to the longitudinal direction, therebyobtaining the desired length to diameter ratio. It appears that materialhaving the length to diameter ratio according to the invention isreadily processable into a solid propellant, while through the treatmentthe sensitivity to shock and/or friction does not or does notsignificantly increase.

According to another aspect of the invention two or more fractions ofsuch crystalline hydrazinium nitroformate can be combined with eachother to form a composition with a multimodal particle sizedistribution. Common particle sizes of crystalline hydraziniumnitroformate according to the invention range between 1 and 1,000 μm.According to the invention a material may then be made which is built upfrom a multimodal fraction of particles having a (number) averageparticle size ranging between 1 and 1,000 μm. It has been found thatsuch combinations lead to considerably higher filling degrees than canbe realized with the starting material or with a multimodal mixture ofdifferent starting materials.

The invention also relates to a method for the preparation ofcrystalline hydrazinium nitroformate having a length to diameter ratioof at most 2.5, which comprises pressing a starting material having alength to diameter ratio of at least 3, for instance 4 or more, under apressure of at most 7 MPa to form a material having the desired lengthto diameter ratio. According to a preferred embodiment a pressureranging between 4 and 5.75 MPa is used.

After this treatment the crystals may optionally be subjected to amechanical post-treatment, for instance to round the sharpest edges ofthe broken crystals. A suitable treatment is the so-called “drumming”,which comprises treating the crystal particles with ceramic balls ofabout 2 to 3 mm in a slowly rotating cylindrical tube.

The invention will now be explained by means of some examples, which,however, should not be regarded as limitative.

Two fractions of crystalline hydrazinium nitroformate were analyzed.Material A had an average particle size (number) of 575 μm and an L/D of5.7. Material B had an average particle size (number) of 100 μm and anL/D of 5.0. Both materials were pressed at a pressure of about 5 MPa.The tap density and bulk density of the untreated and the treatedmaterials were determined and are given in the following table.

Bulk density Tap density HNF type [g/cc] [g/cc] Untreated A 0.83 0.87Untreated B 0.51 0.65 Treated A 0.91 1.11 Treated B 0.65 0.95 Mixturetreated — 1.25 A/B (optimum: 70-65%/ 30-35%)

What is claimed is:
 1. A crystalline hydrazinium nitroformate having alength to diameter ratio of at most 2.5 and a sensitivity to frictionand shock not below 20 N and 2 J, respectively.
 2. A crystallinehydrazinium nitroformate having a length to diameter ratio of at most2.5, obtainable by pressing hydrazinium nitroformate having a length todiameter ratio of at least
 3. 3. A crystalline hydrazinium nitroformatehaving a length to diameter ratio of at most 2.5, but more than 1.5. 4.A crystalline hydrazinium nitroformate according to claim 1 wherein theaverage particle size (d₅₀) ranges between 1 and 1,000 μm.
 5. Acrystalline hydrazinium nitroformate comprising a multimodal particlesize distribution.
 6. A solid propellant based on hydraziniumnitroformate comprising a matrix material having dispersed therein acrystalline hydrazinium nitroformate according to claim
 1. 7. A methodfor the preparation of crystalline hydrazinium nitroformate according toclaim 1 which comprises breaking in a press under pressure needle-shapedcrystalline hydrazinium nitroformate having a length to diameter ratioof at least
 3. 8. A method according to claim 7 wherein the breaking iseffected under a pressure of at most 7 MPa.
 9. A method according toclaim 8 wherein the breaking is effected under a pressure rangingbetween 4 and 5.75 MPa.
 10. A method according to claim 7 wherein thematerial is further treated after pressing.
 11. A method according toclaim 10 wherein the further treatment comprises drumming.
 12. Thecrystalline hydrazinium nitroformate of claim 5, wherein said multimodalparticle size distribution is a bimodal particle size distribution. 13.The crystalline hydrazinium nitroformate of claim 5, wherein saidmultimodal particle size distribution is a trimodal particle sizedistribution.